I agree with Paul with something additional to add. You claim that when
the T-stat is off the 2nd floor would be boiling, and when you turn the
T-stat up, it's cold. All this started after you replaced the T-stat.
Could it be possible that you mis-wired the T-stat and that it is in
"cool" mode. Anotherwords you wired it up as an A/C thermostat so when
you turn down the T-stat, it would actually turn on your boiler instead
of turning it off, and vice-versa when you turned up the T-stat it
would turn off the boiler.
A good way to check is to remove the wires from the t-stat and short
them together. If you start to feel heat coming up to the second floor,
that means that everything is fine, you just have to wire the T-stat
correctly.